The forum is closed?
You’ll just have two groups, and use group permissions to control access to the categories. Full members can see the full categories. Sub members can see the sub categories. Full members who are also in the sub group can see the sub group’s categories. Right?
I’m not sure about skipping required fields. You can turn most of them off. Unless you want one group to fill in fields that the other doesn’t need to, there is no problem.